Some of the "porphyroids" which have grains of quartz and felspar in a finely schistose micaceous matrix are intermediate between porphyries and micaschists of this group. Still more numerous are orthoschists of hornblendic character (hornblende-schists) consisting of green hornblende with often felspar, quartz and sphene (also rutile, garnet, epidote or zoisite, biotite and iron oxides).
Every transition can be found between perfectly normal ophitic dolerites and typical hornblende-schists, and occasionally the same dike or sill will provide specimens of all the connecting stages.
Quincy granite takes a very high polish, owing to the absence of mica and to the coarser cleavage of its hornblende and augite.
Along with these, similarly, hornblende and diabase occur in the Pelew Islands and gneiss and mica 1 These are dependencies of New Zealand, as are also the following islands and groups which lie apart from the main Polynesian clusters, nearer New Zealand itself: Antipodes Islands, Auckland Islands, Bounty Islands, Campbell Islands, Chatham Islands, Kermadec Islands.
The Conway quarries, four in number in 1908, are on either side of the Saco river, south-east and south-west of North Conway; their output is coarse constructional stones, all biotite or biotite-hornblende, but varying in colour, pinkish (" red ") and dark-yellow greenish-grey (" green ") varieties being found remarkably near each other at Redstone, on the east side of the Saco valley.
Magnesium is found widely distributed in nature, chiefly in the forms of silicate, carbonate and chloride, and occurring in the minerals olivine, hornblende, talc, asbestos, meerschaum, augite, dolomite, magnesite, carnallite, kieserite and kainite.
The mountains consist here of an ancient laminated micaceous quartzite, which is in parts a flexible sandstone known as itacolumite, and in parts a conglomerate; it is interbedded with clay-slate, mica-schist, hornblende-schist and haematite-schist, and intersected by veins of quartz.
The other minerals found in the concentrates are pebbles and fragments of pyrope, zircon, cyanite, chrome-diopside, enstatite, a green pyroxene, mica, ilmenite, magnetite, chromite, hornblende, olivine, barytes, calcite and pyrites.
At Inverell in New South Wales a diamond (1906) has been found embedded in a hornblende diabase which is described as a dyke intersecting the granite.
The commonest hornblende is dark brown barkevicite.
It is, however, a hornblende granite and does not possess the mineralogical composition of the syenites of modern petrology.
The rocks are liparites, dacites, hornblende and pyroxene andesites.
Hornblende schists also occur and a compact felspathic rock in the Suris defile.
Quincy granite, a hornblende, pyroxene, bluish or greyish, without mica, was used for the construction of the Bunker Hill monument at Charlestown (in 1826), and of King's Chapel, Boston; and for interior decorations it has found some use, for example in the Philadelphia city buildings.
The gneiss is mostly grey, but occasionally pinkish, its essential constituents (felspar and quartz) being almost always associated with dark mica (biotite) and hornblende in variable quantity.
The rock is therefore a hornblende-granitite-gneiss.
Red and grey sandstones, hornblende and argillaceous schist are found in the mountains, porphyry and basaltic rocks also occur; sulphur and fuller's earth are worked.
The most certain representatives of the Archean are the gneiss and schists of the Dove river and the upper Forth, and the hornblende-schists, which are exposed in the river valleys on the margins of the central plateau.
These sediments are fine and tenacious; their principal components, in addition to clay, being small grains of quartz, zircon, tourmaline, hornblende, felspar and iron compounds.
In addition to the reddish or brownish argillaceous matrix it contains fresh or decomposed crystals of volcanic minerals, such as felspar, augite, hornblende, olivine and pumiceous or palagonitic rocks.